The development of High Velocity Oxygen Fuel (HVOF) spray devices involve the heating of powder particles which are heated and impacted against a workpiece surface to form a coating thereon. Such field is exemplified by my U.S. Pat. No. 5,120,582 issued Jun. 9, 1992 and entitled "MAXIMUM COMBUSTION ENERGY CONVERSION AIR FUEL INTERNAL BURNER." In recent developments within this art, I have learned that the best coatings are produced when the heated particles are in either their solid or plastic state. Further, when in their solid or plastic state, it is possible to increase the temperature of the particles corresponding to the kinetic energy expended upon impact of the high velocity particles against the surface of the workpiece to be coated and to even melt high melting point materials sufficiently to ensure a firm mechanical bond with the surface of the workpiece to be coated. Such basic impact fusion technique has its genesis in U.S. Pat. No. 2,861,900 issued Nov. 25, 1958 and entitled "JET PLATING OF HIGH MELTING POINT MATERIALS."
I have found that fully molten particles are not desired as, upon impact, they splatter with resulting high oxide levels as well as creating voids in the coating. Also, the liquid particles impose a high heat load against the workpiece and in the coating itself, leading to high internal tensile stresses. Thus, only thin coating layers can be produced using liquid particle impingement on a substrate or workpiece, as tensile forces lead to cracking and coating separation from the substrate material.
For these reasons, high-quality coatings, are, now, nearly universally the result of using powdered feed material and by controlling the heat input to the particles to assure that fully fused particles do not result. Supersonic jet velocities are now common, leading to high particle impact velocities against the substrate surface. Such knowledge has resulted from employing the process and apparatus such as that exemplified by my U.S. Pat. No. 5,120,582.
Importantly, when wires or rods are substituted for powder in such supersonic flame jet spray devices (HVOF), the coating material must be melted to produce spray particles as droplets. The molten liquid drops at normally used stand-off distances (gun exit or outlet to Substrate or workpiece surface) remain liquid upon impact and splatter energetically forming porous and oxidized coatings. Conventionally when using the HVOF process and apparatus to spray powdered material, the stand-off distance is usually between 5 and 12 inches. Increasing the stand-off distance is not desirable, since many of the particles lose so much of their internal energy that, upon impact they bounce off the target or remain as solid particles within the coating as it builds on the face of the workpiece, thereby forming a weak spot due to improper adhesion to adjoining particles.
The present invention is premised on the discovery that by doubling, for instance, the normal stand-off distance of flame spray apparatus of the internal burner type using solid wire or rod as the coating material feed which is fed axially through the flame spray gun and into and coaxial with the flame spray itself in contrast to such flame spray apparatus which is customarily used with such gun or internal burner supplied with a coating material in powder form, the increase in the stand-off distance allows time for the liquid particles to lose a sufficient amount of heat and to become plastic, or even solid, rather than molten prior to impact on the face of the workpiece. Under such conditions, upon impact there is little, or no, splattering and the coatings building on the workpiece face appear very similar to their powder-sprayed counterparts possessing high density and strength and having significantly reduced oxide content.